I ran a bandit4 with a JE 59mm kit in for a number of years (road) and had no problems at all. It gave 63bhp with no internal tuning + pipe and carb setup. I found the main reason the JE piston weighed more than stock was due to the pins being a little hardcore, they had a parallel hole through the middle where as the OE pin was machined with tapers to remove weight from the ends. I found it loved to rev and this could also be its downfall.
All in all a nice bit of kit.
If your rods took 16mm pins instead of 15mm you could use FZR6 pistons at 59mm and just get the valve cutouts machined to suit your motor.